Credit Quality (Credit Quality Additional Information) (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Troubled Debt Restructuring
Line of credit commitments for modified troubled debt restructurings	$ 41	$ 24
Letter of credit commitments for modified troubled debt restructurings	58	67
Mortgage loans in process of foreclosure amount	212	$ 153
Commercial aggregate borrower relationship subject to individual review for impairment	$ 1